This invention relates to a method for fabricating a heat mirror for incandescent lamp envelopes and the resulting product and, more particularly, to an improved and economical method for applying Ag/TiO.sub.2 and TiO.sub.2 /Ag/TiO.sub.2 heat mirrors for an incandescent lamp envelope.
The use of heat mirror filters as an envelope coating for incandescent lamps is described in U.S. Pat. No. 4,017,758 dated Apr. 12, 1977 to Almer. The particular filter described is a heavily doped metal oxide filter which has an interference filter coated thereover. More particularly, the doped metal oxide filter is tin-doped indium oxide. In the operation of such a lamp, infrared radiations emitted by the filament are reflected back toward the filament to contribute to filament heating, while visible radiations are passed through the filter, with a resulting improvement in the efficiency of conversion of electric energy into visible light.
The use of tin-doped indium oxide films for use as heat mirrors for solar energy utilization and the use of titania-silver-titania heat mirrors for solar energy utilization are disclosed in Society of Photo-Optical Instrumentation Engineers, Volume 85, pages 39-46 (1977), article by John C. Fan. A similar disclosure is set forth in Applied Optics, Volume 15, No. 4 (April 1976) pages 1012-1017, article by Fan et al.
Transparent heat mirrors of titania/silver/titania for solar energy collection of radiations are described in Applied Physics Letters, Volume 25, No. 12, 15 December 1974, pages 693-695, article by Fan et al. The physical design considerations for such heat mirrors are discussed in detail in these references. Whenever titania is utilized as a heat mirror film constituent, it is disclosed as being deposited with an RF sputtering technique.
An incandescent lamp wherein the envelope is provided with a multi-layer titania/silver/titania heat mirror is disclosed in Lighting Design & Application, Volume 9, No. 6, June, 1979, pages 7 and 8. Such a lamp is described in greater detail in U.S. Pat. No. 4,160,929, dated July 10, 1979 to Thorington et al. This patent indicates that the coating layers can be applied by other than RF sputtering, such as dipping, spraying, vapor deposition, chemical deposition, etc., but no details are given.
U.S. Pat. No. 2,689,858 dated Sept. 21, 1954, to Boyd discloses organic solvent soluble polymers of tetra-organo derivatives of orthotitanic acid. These are prepared by reacting alkyl titanates with a small amount of water to produce a haze-free polymer. These polymers have utility as modifiers for condensation resins.
U.S. Pat. No. 3,460,956 dated Aug. 12, 1969 to Dahle discloses the formation of thin titania coatings from clear alcohol-water solutions which are modified with lactic acid or nitric acid. In the case of nitric acid, the concentration is at least 0.5 mole acid per mole of tetraalkyl titanate placed into the solution.
U.S. Pat. No. 2,768,909 dated Oct. 20, 1956 to Halsam discloses applying tetraalkyl titanate to a substrate and then hydrolyzing same from the moisture in the atmosphere in order to produce a thin film which in some cases can be transparent. A similar technique has been used to coat an incandescent lamp envelope, as disclosed in British Pat. No. 703,127, published Jan. 27, 1954.
U.S. Pat. No. 3,094,436 dated June 18, 1963 to Schroder discloses depositing on a substrate partially hydrolyzed organic titanic and/or silicic acid esters in an alcohol vehicle, which on heat treatment convert to transparent reflection-reducing film coatings.